Automotive cooling assemblies typically include a heat exchanger and a cooling fan for passing ambient air over the heat exchanger. Heat exchangers are usually a type of radiator with a liquid passing therethrough. The liquid flows through the heat exchanger and is cooled by the ambient air.
Cooling fans connected directly to an automotive engine would continuously run at the same rotational speed as the engine. The cooling requirements of the engine may vary over time depending upon any number of factors, such as temperature of the engine, ambient temperature, speed of the vehicle, or load carried by the vehicle. Hence, it is frequently desirable to rotate the cooling fan at different rotational speeds irrespective of the rotational speed of the engine. Accordingly, cooling fans are typically not connected directly to the engine. Cooling fans are connected to a fan drive which interconnects the engine and the cooling fan. The fan drive will rotate the cooling fan at various speeds when needed. There are currently four types of fan drives which are well known in the automotive art, these include viscous fan drives, on-off clutches, hydraulic fan drives, and electric fan drives.
Viscous fan drives typically include a cylindrical housing defining a chamber therein. The cooling fan is mounted to the outside of the cylindrical housing. A rotatable drive shaft and disc extend from an engine and into the chamber. The chamber is partially filled with an oil or similar fluid. When the drive shaft begins to rotate, the disk creates a shearing force on the oil which transmits a torque to the cylindrical housing to rotate the cooling fan. The speed of the cooling fan is varied by changing the amount of oil inside the chamber. U.S. Pat. No. 4,278,159 to Roth et al. discloses this type of viscous fan drive as used in an automotive application. Viscous fan drives, however, have a number of deficiencies. The maximum rotational speed the cooling fan is limited to the rotational speed of the engine. Viscous fan drives rely on centrifugal forces to transfer the oil into the chamber and to create the necessary shearing forces. At low engine speeds there is little centrifugal force to create the necessary shear forces. Hence, the cooling fan may not rotate at a desired speed.
On-off clutches typically connect two surfaces to each other using pressurized air from a vehicle's compressor. Torque is transmitted mechanically by the surfaces contact and friction. The primary disadvantage to this type of system is that the cooling fan is rotating at either full speed, which is equal to the engine speed, or not rotating at all. There is no speed modulation. Further, these systems require a compressed air source which is usually only available on large trucks.
Hydraulic fan drives usually have a pump which drives a hydraulic motor connected to the cooling fan. A valve is used to control how much flow and/or pressure goes to the hydraulic motor which in turn controls the cooling fan rotational speed. These hydraulic fan drives are, however, very expensive to purchase and maintain which generally prohibits their use in most automobile applications.
Electric fan drives are very common and typically consist of an electric motor connected to the cooling fan. By varying the electric power to the motor the rotational speed of the cooling fan may be adjusted. The primary disadvantage to these types of fan drives is the amount of electricity needed to power the motor at high speeds. This amount of electricity can be particularly burdensome when a large motor, such as a truck motor, is idling at a low speeds.